1. Field of Invention
The present invention relates to chemical-mechanical polishing (“CMP”) compositions and methods of making and using the same.
2. Description of Related Art
CMP is a technology that has its roots in the pre-industrial era. In recent years, CMP has become the technology of choice among semiconductor chip fabricators to planarize the surface of semiconductor chips as circuit pattern layers are laid down. CMP technology is well-known, and is typically accomplished using a polishing pad and a polishing slurry composition that contains a chemical reagent and abrasive particles. The chemical reagent functions to chemically react with one or more materials on the surface of the layer being polished whereas the abrasive particles perform a mechanical grinding function.
One of the uses of CMP technology is in the manufacture of shallow trench isolation (STI) structures in integrated circuits formed on semiconductor chips or wafers such as silicon. The purpose of an STI structure is to isolate discrete device elements (e.g., transistors) in a given pattern layer to prevent current leakage from occurring between them. Recent technological advancements that facilitate the fabrication of very small, high density circuit patterns on integrated circuits have placed higher demands on isolation structures.
An STI structure is usually formed by thermally growing an oxide layer on a silicon substrate and then depositing a silicon nitride layer on the thermally grown oxide layer. After deposition of the silicon nitride layer, a shallow trench is formed through the silicon nitride layer, the thermally grown oxide layer and partially through the silicon substrate using, for example, any of the well known photolithography masking and etching processes. A layer of a dielectric material such as silicon dioxide is then typically deposited using a chemical vapor deposition process to completely fill the trench and cover the silicon nitride layer. Next, a CMP process is used to remove that portion of the silicon dioxide layer that overlies or covers the silicon nitride layer and to planarize the entire surface of the workpiece. The silicon nitride layer is intended to function as a polishing stop that protects the underlying thermally grown oxide layer and silicon substrate from being exposed during CMP processing. In some applications, the silicon nitride layer is later removed by, for example, dipping the article in a hot phosphoric acid solution, leaving only the silicon dioxide filled trench to serve as an STI structure. Additional processing is usually then performed to form polysilicon gate structures.
It should be readily apparent that during the CMP step of manufacturing an STI structure on a silicon semiconductor substrate, it would be highly advantageous to use a polishing agent that is capable of selectively removing silicon dioxide in preference to silicon nitride, which is used as the stop layer. Ideally, the rate at which silicon nitride is removed by CMP would be nil, whereas the rate at which the silicon dioxide overlying the silicon nitride stop layer is removed by CMP would be very high. This would allow high manufacturing throughput. The term “selectivity” is used to describe the ratio of the rate at which silicon dioxide is removed to the rate at which silicon nitride is removed by the same polishing agent during a CMP process. Selectivity is determined by dividing the rate at which the silicon dioxide film is removed (usually expressed in terms of Å/min) by the rate at which the silicon nitride film is removed.
It is known that the removal rate of the silicon dioxide trench fill material can be made to be quite high by varying polishing conditions such as increasing pad pressure and using larger abrasive particles in the slurry. However, these polishing conditions also tend to increase the silicon nitride removal rate, which can affect the uniformity of the final silicon nitride layer thickness and can cause other defects, such as scratching, in the final product. Thus, it is important for a CMP slurry composition to promote a reasonable silicon dioxide removal rate while, at the same time, inhibiting or suppressing the rate of silicon nitride removal. This too, however, must be done in moderation for some applications. When the selectivity of a CMP slurry is too high coupled with a very low silicon nitride removal rate, other problems such as “dishing” of the trench silicon dioxide can occur, which can result in severe topography variations once the silicon nitride stop layer is removed. Thus, a CMP slurry composition needs to be able to balance these factors in order to be useful in STI processing.
In the past, polyacrylates and certain amino acids have been added to CMP slurry compositions to obtain highly selective polishing of silicon dioxide in preference to silicon nitride. In most prior art CMP slurry compositions that employ these additives, as more of the additive is added, both the silicon dioxide and silicon nitride removal rate decreases. This can be problematic in some instances where removal rate on silicon dioxide is too slow, thereby decreasing manufacturing throughput on shallow trench isolation (STI) structures.